The following abbreviations which may be found in the specification and/or the drawing figures are defined as follows:
ACK acknowledgement
D2D device-to-device
eNB, eNodeB evolved Node B/base station in an E-UTRAN system
E-UTRAN Evolved UTRAN (LTE)
FDD frequency division duplex
GSM Global System for Mobile Communications
HARQ hybrid automatic repeat request
LTE Long Term Evolution
LTE-A Long Term Evolution Advanced
M2M machine-to-machine
MTC machine-type communication
NACK negative acknowledgement
OFDM orthogonal frequency-division multiplexing
PDSCH physical downlink shared channel
PDCCH physical downlink control channel
PHICH physical HARQ indicator channel
PUCCH physical uplink control channel
PUSCH physical uplink shared channel
RTT round trip time
TDD time division duplex
Tx transmission
UE user equipment
UMTS Universal Mobile Telecommunications System
UTRAN Universal Terrestrial Radio Access Network
WCDMA Wideband Code Division Multiple Access
D2D communications have been the subject of increasing research in recent years. D2D encompasses direct communication among portable devices without utilising nodes/base stations of an infrastructure-based wireless network (typically a cellular network, such as GSM, WCDMA, LTE or the like). D2D communications reduce the load on base stations/wireless networks and also presents new service opportunities. There is a subset of D2D commonly termed M2M (or equivalently MTC) which refers to automated communications from and to radio devices that are not user-controlled, such as for example smart meters, traffic monitors and many other types. Typically, M2M communications are infrequent and carry only small amounts of data compared to cellular communications and D2D communications that are not M2M. To keep costs low, given their more focused purposes, many M2M devices have quite limited capabilities as compared to conventional UEs.
Specific to LTE and LTE-A systems, there has been proposed a study item to evolve the LTE platform in order to cope with the demand of such D2D communications by studying enhancements to the LTE radio layers that allow devices to discover each other directly over the air and potentially communicate directly when viable, taking system management and network supervision into account. See for example documents Tdoc-RP-110706 entitled “On the need for a 3GPP study on LTE device-to-device discovery and communication”; Tdoc RP-110707 entitled “Study on LTE Device to Device Discovery and Communication—Radio Aspects”; and Tdoc-RP-110708 entitled “Study on LTE Device to Device Discovery and Communication—Service and System Aspects”; each by Qualcomm, Inc; TSG RAN #52; Bratislava, Slovakia; May 31-Jun. 3, 2011. Document RP-110106 describes one of the main targets is that the “radio-based discovery process needs also to be coupled with a system architecture and a security architecture that allow the 3GPP operators to retain control of the device behaviour, for example who can emit discovery signals, when and where, what information do they carry, and what devices should do once they discover each other.”
One 3GPP working group is currently discussing and defining use cases and service requirements for the D2D. Such use cases include social applications, local advertising, multiplayer gaming, network offloading, smart meters and public safety. Specifically, social applications can use D2D for the exchange of files, photos, text messages, etc, VoIP conversations, one-way streaming video and two-way video conferencing. Multiplayer gaming can use D2D for exchanging high resolution media (voice & video) interactively either with all participants or only with team members within a game environment. In this gaming use case, the control inputs are expected to be received by all game participants with an ability to maintain causality. Network offloading can utilise D2D when an opportunistic proximity offload potential exists. For example, a first device can initiate transfer of a media flow from the macro network to a proximity communications session with a second device, thereby conserving macro network resources while maintaining the quality of the user experience for the media session. Smart meters can use D2D communication among low capability MTC devices, for vehicular communication (for safety and non-safety purposes), and possibly also general M2M communication among different capability devices/machines. In the public safety regime, there can be either network-controlled D2D or a pure ad hoc D2D which does not utilise any network infrastructure for setting up or maintaining the D2D links. These are the two main categories of D2D networks, one taking place under control of a controlling (cellular) network and typically using licensed spectrum, and the other being ad hoc D2D which can work autonomously without network coverage.
In the cellular-controlled approach, the discovery communications, by which devices can discover each other's presence, are likely to be multiplexed with the (normal) cellular communications taking place on the same radio resources. However, it is important to ensure that these discovery communications do not interfere with or otherwise affect the operation of other devices using the cellular network with the (normal) cellular communications that conventionally take place, whether these other devices are also involved in D2D communications or not.